Power semiconductor modules comprise one or a plurality of power semiconductor switches. The advances in the power semiconductor materials with a high band gap that are used for the power semiconductor switches allow the production of unipolar components for high reverse voltages (e.g. 1200 V) such as would be possible in silicon (Si) only with high on resistances being accepted. Unipolar transistors that stand out include junction field effect transistors (JFETs), for example, particularly those whose semiconductor body comprises the semiconductor basic material silicon carbide. They have a high blocking capability in conjunction with fast switching behavior.
FIG. 1 shows a circuit symbol of a junction field effect transistor J1. The junction field effect transistor J1 has a first load terminal 11, a second load terminal 12 and also a control terminal 13. A load path is formed between the first load terminal 11 and the second load terminal 12, which load path can be driven by the control terminal 13, such that the load path is fully or partly turned on, or else turns off fully. Hereafter, a semiconductor component or a circuit arrangement is referred to as “in the off state” if its load path turns off. Accordingly, a semiconductor component or a circuit arrangement is referred to as “in the on state” if its load path turns on.
In the case of the junction field effect transistor J1 shown, the first load terminal 11 forms the drain terminal, and the second load terminal 12 forms the source terminal. The control terminal 13 is also referred to as the gate terminal. Furthermore, the junction field effect transistor J1 can have an optional body diode 14 integrated into its semiconductor body.
In this case, the load path is driven by a suitable electrical drive potential being applied to the control terminal 13. This can be effected, for example, by means of a control voltage between the control terminal 13 and the second load terminal 12. If the control voltage is equal to or almost zero, then the load path is in the on state. In order to put the load path into the off state, the control terminal 13 has to be sufficiently negatively biased relative to the second load terminal 12, such that the current channel in the semiconductor is fully pinched off. The junction field effect transistor J1 is therefore also referred to as normally on.
A normally on behavior is not always desirable, however, from the point of view of the user. Therefore, in many applications, rather than normally on junction field effect transistors, normally off transistors are used such as e.g. enhancement-mode MOSFETs or IGBTs, that is to say transistors whose load paths are switched off without an external voltage being applied between the control terminal and the source or gate terminal.
Only significantly lower reverse voltages can be achieved with the normally off unipolar transistors available nowadays, however, in comparison with junction field effect transistors based on silicon carbide. Secondly, the bipolar transistors available nowadays have poorer dynamic properties.